Study Reviews and Rethinking the Key Processes for Managing Building Materials to Enhance the Circular Economy in the AEC Industry

Abstract

The rapidly accelerating economic development of newly industrialised countries (NICs) has created far-reaching environmental problems. The new construction of numerous infrastructures and buildings, particularly in the architecture, engineering, and construction (AEC) industry, has led to an exponential increase in the demand for raw materials and energy, which is leading to the depletion of natural resources. The approach to treating these buildings at the end of life has also raised concerns worldwide. Transforming the current linear development model into a circular economy is considered an effective solution. This paper reviews a broad range of relevant literature, extracting four key factors influencing building circularity (BC) from past studies. These factors are interpreted as four key processes dealing with building materials: pre-treatment, composition, decomposition, and post-treatment. We demonstrate how materials are treated throughout the building lifecycle to illustrate the interrelationships among these processes and to exemplify the potential of the key processes for effecting BC. Additionally, two examples are used to support the theoretical framework. This study intends to make contributions to circular economy theories and to provide references for policymakers and practitioners.

1. Introduction

Newly industrialised countries (NICs), such as China and India, have experienced rapid economic growth in recent decades. Simultaneously, cities have expanded enormously due to substantial urban migration from rural areas. For example, China’s urbanisation has increased from 17.9% to 64.7% since 1979, the year the Organisation for Economic Co-operation and Development (OECD) first introduced the concept of NIC [1,2]. Swift urban growth and building in these countries have caused the production of vast quantities of construction demolition waste (CDW) [3]. China, for example, generated approximately 2500 Mt of CDW in 2015 [4], while Brazil accumulated approximately 45.1 Mt in 2016 [5].

It is crucial to recognise that 90% of CDW comes from the demolition of built structures, which generates 39 times more waste per square metre than construction [6]. Previously an unrecognised problem, the management of demolition wastes (DW) has become a global issue across the entire architecture, engineering, and construction (AEC) industry [7]. DW processing capacity is regrettably inadequate, and the required management systems are fragile, especially in the NICs [8]. Illegal dumping of DW is common throughout India, Brazil, and other industrialising countries [9]. Meanwhile, DW is overwhelmingly treated through dumping and landfilling in China, for example [10], with only 5% of the total treated as recoverable [11]. Regardless of landfilling or dumping, current practices cannot be sustained. Typical AEC methods consume vast quantities of arable land resources, even increasing the risks of landslides and other catastrophes [12,13], and additionally generate large amounts of toxic substances in DW streams, causing severe soil and water pollution [14]. The creation of large numbers of new buildings and infrastructures leads to significant increases in the consumption of building materials and energy [15], escalating resource scarcities and related expenses. Furthermore, space for DW landfills is already becoming limited and will be severely constrained if such development, construction, and demolition practices are allowed to continue [16].

Studies have increasingly demonstrated that DW problems are the result of a linear economy (LE), which is unsustainable, following a one-way, ‘take-make-consume-dispose’ development pattern [17]. To fundamentally address DW problems, the concept of a circular economy (CE) has been gradually conceived to replace the unrealistic linear development model [18,19]. CE shares some features with LE, such as extracting raw materials and manufacturing products. However, the most significant difference between the two models lies in how waste is treated. In the ideal model of the CE system, CDW wastes are converted into resources and reused indefinitely as renewed materials, preserving their value as resources and reducing the requirement for new raw materials. CE aims to apply a closed-loop system that uses all resources as many times as possible [20]. This concept has already received the attention of policymakers in some countries and regions, such as Germany, Japan, and China [21]. For example, China approved the Circular Economy Promotion Law (CEPL) in 2008, in which only two paragraphs of the law explicitly refer to the AEC industry. One is paragraph 23, where it is recommended to adopt energy and material saving techniques and to use nontoxic building materials. Another paragraph, paragraph 33, suggests that construction corporations treat CDW through comprehensive utilisation during the construction process. Furthermore, supporting legislation for CEPL remains to be clarified. For example, the law does not stipulate responsibility for implementing the law or how recycling would occur [22]. More importantly, specific targets and evaluation criteria for the degree to which the circular economy should be implemented across the AEC industry have yet to be developed. Thus, specific methods for implementing and measuring building circularity (BC) remain unknown [23]. A crucial impediment to achieving a CE lies in the central fact that BC is inextricably bound to interdisciplinary practices throughout the AEC industries and the development of realistic methods for targeting and evaluating CE goals at all levels. Although single factors and influencers have been widely investigated in recent years, the vital interrelationships among them and their collective contributions to building sustainability must still be clarified.

For the above reasons, this study aims to deeply examine key processes for managing building materials in a circular construction system. By reviewing past studies, making logical deductions, and analysing typical design and construction details, this paper attempts to explain multivariate influences on BC within the lifecycle, especially the relationships of key processes and collaborative working mechanisms. Based on the above analyses, suggestions are made for architects, contractors, workers, and other AEC practitioners in the future. Moreover, this research provides basic arguments for NICs with scientific references to assist in guiding CE decision-making and policies for embedding CE practices across the industries related to the AEC industry.

2. State-of-the-Art

2.1. Theories and Tools Related to BC

As CE became popular in the West, several new concepts emerged. For example, the Cradle to Cradle (C2C) cycle operation system was proposed by the American architect McDonough and the German chemist Braungart in the 1990s. They illustrated this sustainable development model by describing the growth pattern of cherries [24]. Furthermore, McDonough and Braungart provided a theoretical description of C2C and proposed a certification procedure in Cradle-to-cradle, Remaking the way we make things [25]. C2C is not a one-way linear development model from growth to extinction but a cyclical development model. The model includes the following elements: (a) Eliminating the concept of “waste”. Designing products and materials that can be reused many times. Creating systems that collect and restore the value of materials after initial use; (b) Relying on renewable energy to maximise utility; (c) Respecting people and ecosystems and promoting diversity. This implies that CE and C2C are holistic frameworks aimed at transforming a single linear development paradigm into a sustainable, circular development model [18]. How these theories can be applied to the AEC industry and achieve circular construction largely remains at the exploratory stage.

In addition, Markova and Rechberger proposed the notion of a material passport (MP) to support CE [26]. MP conceived a tool for recording and tracking materials, products, and circularity potential by using information technologies (ITSs) and flexible building design. It aims to accelerate the construction of cities with more circularity [27]. Materials can be identified, removed and reused multiple times, tracked in a database through MP [28]. In recent years, some investigators have been looking at how MP can be used to lower financial barriers to promote material reuse for the benefit of stockists and fabricators [29]. Other researchers have used MP to calculate building indicators in the construction, operation, and end-of-life phases [30]. It is possible that MP could store and provide building materials’ information and, thereby, offer help at the design and material manufacturing phase. In short, MP is a research tool proposed to track the transformation process of all materials, including construction materials, based on their specific properties. However, due to insufficient data on the available building stock for the effective management of end-of-life materials [31] and the fact that information is often incomplete or not available to professionals promptly at all times [32], MP remains a concept still waiting for more advanced methods that would allow wider realistic use across the AEC industry.

Other tools are used in the design phase to improve BC. In the late 1970s, Boothroyd and Dewhurst performed a series of studies on design for assembly (DfA) directions for studying assembly methods and costs in the design stage [33,34]. The use of DfA brought many benefits, such as reduced installation and manufacturing costs, improved quality and product simplification [35]. As the concern for environmental issues grows, so do studies that consider disassembly and recycling issues starting at the design stage, known as Design for X (DFX) [36]. Subsequently, the design concept for disassembly (DfD) was taken from DFX and applied to architecture. In Technology, Design and Process Innovation in the Built Environment, Philip Crowther mentioned that after a building was demolished, most materials suitable for recycling were downcycled, and very little was reused. It remains most often physically impossible to successfully recover or reuse materials or components during building deconstruction since building design is not usually built with recycling in mind. Therefore, it is necessary to use DfD to improve the reuse of materials and components after building deconstruction [37]. However, DfD principles are more concerned with the reuse of building components and are lacking in the recycling of construction materials [38].

Life cycle assessment (LCA) is a method for calculating the environmental burden and possible impact of a product across its entire life cycle [39]. LCA has been used in the AEC industry since 1990 and has gradually become an important tool for assessing buildings [40]. Examples include using LCA to evaluate CDW’s management system [5], comparing the environmental benefits of reuse and recycling [41], and comparing the environmental impact of manufacturing products using raw materials or secondary materials [42]. However, LCA is predominantly used to analyse individual products or a single life cycle, but in a building system that operates on the principle of CE, multiple cycles should occur at different periods. As a result, the LCA methodology must be modified to provide a more comprehensive and adequate study of the AEC industry’s adherence to CE principles [43].

2.2. Past Studies on BC Influencing Factors

Building materials are one of the influencing factors that draws great interest to BC research worldwide. In the study of materials for upgrading sustainability, Finch, Marriage, Pelosi and Gjerde [44] used a hybrid method, combining a literature study with qualitative observations and interviews, to completely examine possibilities for adopting material recycling in the AEC industry. Sassi [16] created a system of criteria to encourage construction that produces less waste with reduced environmental effects. This criterion is named the closed-loop material cycle (CLMC), in which building materials and related elements play an especially important role. However, the CLMC does not consider construction activities themselves. A study by Heisel and Rau-Oberhuber [30] evaluated the capabilities of Material Passport and Madaster (a digital platform to document, register and archive the materials applied in buildings and construction objects) to document the inventory and flow of materials in a circular building environment as well as its potential as a design tool for promoting the drive towards recycling. The authors reported that the continuous development of tools and systems for tracking materials is a critical prerequisite for transitioning to a circular development model in the AEC industry. Choosing construction materials and how they are designed for use are often the focus of architects when exploring effective strategies to make the AEC industry sustainable [45].

Beyond building materials, specific types of construction are also considered among the influencing factors in BC evaluations, as reported by the Ellen MacArthur Foundation [46]. Although the fundamental connection between building materials and construction practices is vital to achieving BC, this has triggered little attention in the construction industry, while some researchers have focused on its importance. Braakman, Bhochhibhoya and de Graaf [47] emphasised that materials are linked with construction and that buildings with specific connections between materials and components would improve the potential for circular construction. For example, dry-connected floors and other components, such as click-based brickwork, inner wall panels, and tile panels, can be used to replace poured-in-place solid floors and walls. Mayer [48] argued that constructive optimisation of the assemblies within building components could improve sustainability. As an example, they used typical timber-framed external walls and redesigned assembly joints specifically to facilitate material recovery performance, with the result that the newly created method had a 35–47% improvement in materials recovery compared with standard assembly methods.

Meanwhile, research in the field of building deconstruction continues due to its central role in BC [49]. The deconstruction process, distinguished from traditional demolition, allows for a higher level of material recovery; not only is less material discarded, but the need for virgin resources is also reduced [50]. Bertino, Kisser, Zeilinger, Langergraber, Fischer and Österreicher [51] proposed a strategy for construction and reuse to provide more sustainable solutions for the end-of-life of buildings in which deconstruction allowed the treatment of separated objects for different possibilities. Schultmann and Sunke [52], considering deconstruction as a central tenet of sustainability, developed an integrated deconstruction-recovery model and introduced a method for assessing the environmental impact of product recovery.

Waste recovery critically affects the development of a circular economy in the construction sector, and Sagan and Sobotka [53] reported that waste recovery is a key factor of CE. They used the decision-making trial and evaluation laboratory (DEMATEL) method to investigate the significance of waste recovery in an assumed circular development. Gálvez-Martos, Styles, Schoenberger and Zeschmar-Lahl [54] reviewed national management practices for handling CDW in the European Union (EU) countries. They found that decision-making chains varied, particularly in the areas of waste treatment and the development of markets for secondary materials. Their research provided a basis for developing CDW management policies, strategies, and circular economy solutions for the AEC industry by systematically documenting the current best practices observed in Europe. Kartam, Al-Mutairi, Al-Ghusain and Al-Humoud [55] described the status of the CDW treatment system in Kuwait and its potential problems for the environment, humans, and the economy. Their study also examined the feasibility of solutions for managing and controlling CDW in a cost-effective and environmentally safe manner.

In addition, the use of transportation logistics has been suggested as a contributing factor to waste recovery efficiency [56]. Stakeholders’ perceptions, decisions, and motivations also influence the reuse of recycled CDW products [57]. From a broader perspective, Sassi [58], Kostakis and Tsagarakis [59] suggested that factors such as the economy, environmental taxation grades, and legislation all affect the overall achievability of a circular economy.

2.3. Research Gap

Tools have been developed to help the AEC industry develop BC systems, as mentioned above. However, the tools can work effectively only in an advanced cyclical development system, yet most countries worldwide, especially NICs, are only in the early stages of conceiving and shaping practical CE systems. Thus, these tools have not been widely used thus far. NICs are exploring mechanisms for adjusting the linear operating practices of the AEC industry to a circle model. Among many factors, this includes identifying constraints and drivers for the industry’s sustainability within the consideration of the building lifecycle.

After reviewing the relevant literature, we found that, whether analysing linear or circular practices, the root determinant for developing BC systems for the AEC industry ultimately rests on building materials and the flow of their forms (Figure 1). As noted by Towa, Zeller and Achten [60], among the comprehensive input of materials used in construction, the proportion recovered and reintroduced represents the measure of BC. The greater the amount of materials involved in multiple physical or chemical conversions throughout the processes of manufacturing, construction, and deconstruction, the closer the system approaches BC. Therefore, each modification is a direct result of corresponding processes, and later processes are influenced by those that preceded them. If the aim is to reduce waste generation—reducing or retarding the transformation of material from natural resources to waste—the focus must be to improve and promote material handling processes. Essentially, identifying the most effective techniques for converting raw resources into construction materials and allowing for their eventual deconstruction into usable wastes is the primary task.

As previously presented, based on a review of existing research, there are few findings that clearly illustrate material treatment processes in the context of bona fide BC. Questions such as how closely these processes relate to BC, in what way practices operate interactively, and by what means the circulation measurably grows must be clarified. This paper, therefore, aims to analyse and summarise this issue by way of theoretical derivation.

3. Methodology

Multiple BC influencing factors were compared by reviewing and analysing relevant studies. The factors are clients, designers, legislation, regulatory bodies, transportation, energy consumption, and logistics etc. [61]. Four key factors were ultimately identified: materials, construction, deconstruction, and waste management (Table 1). Studies that focus on materials or waste management use the substance as a theme. Research on construction and deconstruction usually focuses on the process of building construction or dismantling. As these four factors do not refer to the same category of research objects, this differentiation makes it difficult to consistently categorise the four factors in a comparable context. In accordance with the abovementioned research gap regarding material treatment, this study explores changes in materials during the building lifecycle: manufacturing, construction, dismantling, and recovery, and converts the four key factors into four key processes: pre-treatment, composition, decomposition, and post-treatment. Through this interpretation, we focus on building materials as the single analysed object in four different sequential processes (Figure 2).

Considering natural resources as the starting point and the building lifecycle as an analysis period, the study then reinterprets the four key processes and explores their impact on building sustainability in relation to materials. The specific methods are as follows: (1) To analyse the pre-treatment process, the influences of this process on the material recycling potential are investigated by tracing them back to their sources, using the most common construction materials as examples. (2) In the analysis of material composition, the types of connections of building products or components are classified according to their reversibility. With the help of this classification, the effects of the connections on deconstruction processes are noted, which lead to recommendations for facilitating the dismantling of buildings, including components, products. (3) Similarly, after reviewing a number of studies and examples of building material decomposition, typical methods of deconstruction are categorised according to the methods and conditions. Several strategies are then proposed for optimising the deconstruction process to facilitate material recovery. (4) Finally, analysis of the post-treatment process is described in detail, and a judgement on the destination of the material flow after conversions is suggested. Therefore, the recycling potential and the manner in which materials can be returned to a new building lifecycle are illustrated.

Based on the theoretical analysis, two examples are used to justify the logical account of key processes. The first example relates to concrete components. It focuses on the interactions between composition and decomposition and the role they play in the flow of materials through the building lifecycle. The second example examines the construction details of five walls collected from five relevant studies. Based on the materials and the construction methods used, the potential separation methods are analysed, and a possible form for the final recovery of the material is inferred.

4. Key Processes in Managing Building Materials

4.1. Pre-Treatment

Raw materials are typically processed into building products before they are used in construction. This process, from natural resource to product, is referred to as pre-treatment and focuses on the initial transformation of the material to prepare for construction. For example, logs are processed into conventionally sized wood sheets and then bonded together with a durable, moisture-resistant structural adhesive to form glued laminated timber (GLT). Engineered wood products, such as particle board (PB), oriented strand chipboard (OSB), and medium-density fibreboard (MDF), are made from wood with adhesives (Figure 3). Another example is the smelting of the iron ore used to make metal construction products such as steel bars, bolts, and nails.

Although this conversion process is virtually irreversible, almost all these building products have the potential to be recycled [64,65,66]. For example, MDF can be industrially milled and then reprocessed into PB [67], and steel beams can be remanufactured into reinforcing bars [66]. However, when some toxic materials are used to enhance the performance of building products, their recycling potential can become limited. For instance, wood treated with chromated copper arsenate (CCA) and alkaline copper quaternary (ACQ) preservatives. The metals contained in CCA and ACQ contaminate remanufactured products, and therefore, these woods cannot be recycled [68]. The combustion process of CCA-treated wood emits harmful gases that can be inhaled, causing acute and chronic arsenic poisoning (Figure 3). It cannot be treated by incineration and is usually disposed of in landfills [69,70]. The use of toxic materials significantly reduces the recycling potential of wood and should be avoided as much as possible.

Although concrete technically has the potential to be recycled, the aggregates (gravel of different diameters) and cement (iron ore powder, clay, limestone) that make up concrete are non-renewable resources [71,72]. In addition, cement is among the most energy-intensive materials used in the AEC industry, consuming large amounts of fossil fuels and causing the production of air pollutants such as carbon dioxide, sulphur dioxide, and nitrous oxide in the manufacturing process [73]. Steel is also recyclable, but its raw material, iron ore, is not renewable, and emissions of air pollutants are also a product of the smelting process (Figure 4). In addition, the mining of iron ore can potentially cause metal contamination of groundwater [74].

Obviously, such raw materials may have a direct impact on the recycling potential of building products, require excessive energy consumption, yield airborne pollutants, including CO₂, increase the quantity of the carbon footprint, or impose other significant strains on the environment. Therefore, it is recommended that architects in the design phase minimise the use of products that are manufactured from non-renewable materials or that require significant energy consumption in their production and avoid the use of non-recyclable building products that contain toxic substances.

4.2. Composition

The composition process combines single building materials, products, components, and modules to a finished construction. The reuse of building materials and components is not directly influenced by the composition. This is reflected by the separation results after deconstruction. In other words, sustainability is determined by the material’s reversibility or ability to be converted into a reusable state. According to this analysis and with reference to the classification of whether elements can be completely separated from each other [64], the composition methods can be broadly classified into two types: physical- and chemical-fixed connections. Physical-fixed connection refers to the way materials or components are held together by force, such as nails or bolts. Such elements could be completely detached during deconstruction. Through chemical fixing, the material is usually changed at the molecular level in the composition process and is bound together as an ensemble. Such elements are difficult to separate from each other. Using this classification as a criterion, several conventional connections are listed in Table 2.

Physical-fixed connections are generally considered more conducive to building deconstruction than chemical fixing [85]. However, the difficulty of deconstruction varies widely for physically fixed methods as well. For example, nail fixing and riveted fixing are more challenging to remove than screw fixing [86]. Timber components connected by nails and staples are more complex to separate than those joined by friction or bolts and nuts [48]. Although chemically fixed connectors limit the separation of materials, they do not affect the recycling potential of materials if they are used only between building products made from the same raw materials. Additionally, wherever possible, it is suggested to use connecting materials that are weaker than the elements being linked. For example, mortar that is weaker than bricks can be used to build walls so that it is easier to break apart when dismantled [87]. Thus, the choice of connecting materials and methods greatly affects deconstruction capabilities and practices.

4.3. Decomposition

The process of breaking down a building into smaller units (room modules, components, products, etc.) with material separation is called decomposition. The final states of dismantled buildings by different methods can be divided into two categories: a mixture of multiple materials and single building elements. The former mostly result from heavy mechanical demolition, while the latter usually result from deconstruction work using manual, light mechanical, or electrical tools. Therefore, the process of decomposition can be classified as soft stripping or hard stripping depending on the methods used. Soft stripping is relatively costly and time-consuming, but the functionality of the resulting components or products can be maximised. Hard stripping is highly destructive to building materials and components, but it is less costly [88]. Table 3 lists some of the common tools used and their functions in the decomposition processes.

Soft stripping is promoted because it makes the building components or products easier to reuse or recycle than hard stripping [89]. However, a lack of systematic deconstruction training can be a major obstacle. Workers should be trained in basic operations, large equipment control, hazardous material recognition, fall protection, rescue procedures, etc. [90]. These trainings not only ensure workers’ safety but also increase deconstruction efficiency and reduce labour costs.

As an accelerator for the replacement of hard stripping with soft stripping, the development and application of separation-related tools for building deconstruction play a significant role. The advantages of machine-assisted separation are obvious, including speed, safety, control, accuracy and transparency of work progress [91]. Most importantly, it can replace humans to split up complicated connected components, increasing the usable proportion of individual products or components after deconstruction [92].

Generally, physically fixed connections allow for soft stripping, whereas chemically fixed connections can typically be separated with hard stripping. For instance, bolted steel beams and columns can be detached by removing the bolts using a wrench. Another example is blasting, the most common way to dismantle high-rise buildings constructed of cast-in-place concrete [93]. It is important to note that methods may exist for separating components typically considered inseparable. For example, mid-infrared wavelengths could theoretically remove epoxy resin that is sensitive to its wavelength [94].

4.4. Post-Treatment

The process follows building deconstruction when all the materials have a designated destination, which refers to extending life, entering the next lifecycle or ending life and leaving the lifecycle. This process is called post-treatment. This stage includes sorting, assessment, manufacturing, and alternative designations, such as reuse, recycling, energy recovery, and disposal. Sorting classifies decomposed materials into room modules, components, pure materials, mixed materials, etc. Polluted elements are removed during this process [95]. Subsequently, the remaining materials are analysed and tested to determine their properties. For example, the shear strength of timber beams and the tensile strength of steel are tested to determine whether they can be used again. In addition, the destination of these elements is considered in relation to the corresponding ‘recipient’, for example, determining whether components for reuse exactly fit to a new building functionally and dimensionally. If complex modifications on components are compulsory for reuse, downcycling might be more appropriate based on energy consumption. By comparison, these materials will be treated in a way that maximises environmental benefits.

When components or products exactly match all requirements of the new buildings, they can be reused directly. However, some materials require minor modifications to work properly, which is called adaptive reuse. Defective elements that still have recycling potential can be used as raw materials to manufacture new products. For example, mineral wools separated from CDW are added in the production of gypsum products to enhance their flexural strength [96]. These treatments are considered good environmental practices. They offer great potential for improving and increasing resource efficiency in the AEC industry, thereby reducing energy use and associated carbon emissions [97]. Alternatively, materials that can be converted at high temperatures or through landfills into electrical or thermal energy can be returned to the building lifecycle in the form of energy. These include the incineration of combustible materials instead of fossil fuels or the collection of methane from landfills containing specific materials [98]. In contrast to the abovementioned treatments, unmanaged or haphazard disposal, including deposition of DW on open sites or direct burning, is the most inexpensive but least environmentally friendly method. Figure 5 illustrates the details of the processes of material post-treatment.

It is advantageous to reuse room modules, components, pure building products, and mixed materials after soft stripping. The former three have been examined for the possibility of reuse. However, most buildings are dismantled by hard stripping, and mixed materials are crushed to secondary aggregates in various applications. The transformation of these materials is a process of downcycling. For example, concrete is mostly recycled as low-quality aggregates in road construction. However, some developed countries, such as Belgium and the Netherlands, are already facing a saturated market for low-quality aggregates, with supply outstripping demand [99,100].

To prevent this from happening to the NIC in the future, the most immediate solution should be upgrading remanufacturing technology. This minimises downcycling and simultaneously increases the reuse of different types of materials. There is also a need to improve the standards and legal framework for using secondary materials. Standards and norms ensure the quality of products, while legal systems enable the government to supervise and encourage active participation in recycling by relevant practitioners [101]. In this way, the dual effect of expanding the market and guaranteeing quality will avoid the downcycling of materials and increase the overall proportion of materials reintroduced into the recycling system.

5. Further Analyses with Examples

5.1. Concrete Components

Common concrete structures consist of prefabricated concrete beams and columns assembled on construction sites. The assembly process can be conducted with the use of steel plates, top angles and steel threaded rods (physically fixed) or by casting the connected parts in place (chemically fixed) (Table 2). When physical fixing is used, either soft or hard stripping can be applied during deconstruction. However, hard stripping is essentially the only method of dismantling buildings made of cast-in-place concrete [102]. This example shows that connection types may impose restrictions on separation methods.

Prefabricated concrete components can be reused after soft stripping [103]. However, if the components are hard stripped, their characteristics are typically compromised [88]. Crushed concrete and steel can be sorted out and sent to factories for further processing into new products. Although crushed concrete can be recycled for road construction [104], the disposal of concrete waste is still widely practised in NICs such as China, India, and Brazil [9,10]. As discussed, the specific connection methods used for material composition limit the separation methods used in decomposition, which is correspondingly related to the choice of post-treatment methods (Figure 6). Although these three processes occur in different stages of the building lifecycle and span long periods of time, they are so closely interlinked that they produce a quasi-upstream controlling effect.

5.2. Wall Details

In the following section, five different wall details (A–E) reported in past studies are used as examples to describe the main materials and connections used for creating walls and to simulate the separation methods and waste treatments (Table 4). Wall A is made by using epoxy resin adhesives to bond timber board, oriented strand board (OSB), and extruded polystyrene board (XPS). Wall B is composed of a timber frame with mortise-tenon joints and a brick wall. Wall C is made up of a steel frame filled with polystyrene filler-boards and cast concrete on both sides. Wall D consists of screws joining the concrete board, potter board, steel frame, calcium-silicate board (CSB), and thick steel covering. Wall E has a timber frame with sprayed hemp concrete on one side and sand and lime coatings on each side.

Walls A, C, and E are constructed using chemical-fixed connectors, making the latter separation of the materials extremely difficult and leaving hard stripping as the only choice for decomposition. For Wall B, the beams and columns joined by the mortise tenon can be separated by hand, while the masonry can be pushed directly using a bulldozer. By removing the screws that connect all the elements together, Wall D can be decomposed into individual, reusable materials.

Soft stripping allows for the reuse of the wooden beams and columns of Wall B, as well as the concrete board, steel frame, and steel covering of Wall D. The waste generated from the hard stripping of Wall A is mostly wood of various grain grades, which can be recycled as fuel. However, the waste after demolishing Wall E is composed of concrete, sand, lime, and minor amounts of wood. Thus, the mixture would likely be non-combustible with a low potential for recycling as an aggregate. Perhaps it can be handled through managed disposal. Similarly, Wall C would deconstruct into concrete mixed with polystyrene and steel after decomposing Wall C, with the steel more easily sorted out to be recycled (Table 5).

These five examples explain how the choice of materials and method of constructing walls constrains the methods used for decomposition, which in turn influences post-treatment. Furthermore, the potential for material recovery differs, for example, concrete and steel (Wall C), as well as wood for combustibles in Walls A, C and E. When wood is the primary component of the waste mixture, it is possible to gain energy through incineration (Wall A). However, waste mainly consisting of non-combustible materials is much more difficult to manage.

6. Discussion

6.1. Material Changes in Building Operation

In this study, our analysis of the processes of material treatment did not consider changing practices during the building operation phase. As the operation of buildings during their lifecycle is normally much longer than the other phases, the causes for changes in materials and other factors during this phase could not be easily determined. At the architectural design stage, it is challenging to predict the causes driving design, such as normal business needs, natural disasters, including earthquakes, floods, tsunamis and mudslides, and violent human damage, including arson and vandalism. Even predictable factors, such as the carbonation of concrete and oxidation of metals, influence building materials significantly over time and through environmental shifts.

Moreover, the operation of buildings may not be related to the processes of construction (composition) and decomposition. In other words, the materials are often affected by a superimposition of factors during the operating phase, which has nothing to do with material changes during the composition or the choice of separation methods during decomposition. Considering a steel frame as an example, although welded frames are more prone to brittle damage under earthquake action than bolted frames [110], the generation of such damage is also influenced by the magnitude of nodal stiffness, strength and duration of the seismic wave. Regardless of the degree of damage suffered, bolted connections still provide more options for soft stripping than welding does. The relationship between connection and separation is not disturbed, although the building materials may experience various changes during operation.

6.2. Contributions of the Study

This study used CE and C2C theories as a starting point, which provided a general direction for examining sustainable development trends in the AEC industry. The theoretical framework built in this paper, based on the transformation of materials, can be applied to MP to track information on materials and to supplement material information at the end of life in the building life cycle. This paper extrapolates the logic of the flow of building materials through the building lifecycle as a complement to the DfD design principles in the reuse of recycled materials. In addition, LCA has been a recommended tool by many academics to calculate the impact of building construction on the environment. However, traditional LCA needs to be updated for a circular system to accommodate new situations in which building materials may be reused. In fact, some advanced LCA methods used in developing the CE system have already been invented, such as the circular footprint formula approach and the linearly expressive approach [43]. These methods can predict environmental effects after material treatments, thus helping decision-makers, such as architects and contractors, make choices that are beneficial to the more sustainable development of buildings and to the environment.

7. Conclusions

Based on many studies on CE, this paper used the building lifecycle as a timeline to analyse the principal factors that directly and indirectly affect BC by interpreting these factors into four processes for managing building materials: pre-treatment, composition, decomposition, and post-treatment. The four processes are then examined in depth or categorised to explore changes in building materials, as well as their impact on BC. We found that most building products have the potential to be recycled, but this potential is heavily limited by toxic additives and the use of recalcitrant elements. The way materials are connected is inextricably linked to the ways in which they can be separated and, in turn, affects the final designations and methods for the management of deconstructed building material flows. Two examples are used to substantiate the above theoretical inferences.

This study analysed respective processes within the building lifecycle in a theoretical framework. Further studies could explore the logical interweaving of emerging building materials in ongoing construction processes. Such studies can assist NICs and others in moving towards a circular development system including much-needed policies for implementing CE actions to improve the sustainability of the AEC industry.

Author Contributions

Conceptualisation, H.H. and L.L.; methodology, H.H.; resources, L.L.; writing—original draft preparation, L.L.; writing—review and editing, H.H.; visualisation, L.L.; supervision, H.H.; project administration, H.H.; funding acquisition, H.H. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Material flows in a linear versus a circular economy system.

Figure 2. Interpretation from key factors to key processes.

Figure 3. Pre-treatment of wood-based products.

Figure 4. Pre-treatment of concrete and steel products.

Figure 5. Processes of material post-treatment.

Figure 6. Interlinkage between material composition, decomposition and post-treatment.

Table 1

Previous research and focus.

Literature	Research Focus
	Material	Construction	Deconstruction	Waste Management
Finch, Marriage, Pelosi and Gjerde [44]	√
Sassi [16]	√
Heisel and Rau-Oberhuber [30]	√
Foundation [62]	√	√
Braakman, Bhochhibhoya and de Graaf [47]		√
Mayer [48]		√
Kanters [49]			√
Akinade, Oyedele, Bilal, Ajayi, Owolabi, Alaka and Bello [50]			√
Bertino, Kisser, Zeilinger, Langergraber, Fischer and Österreicher [51]			√
Schultmann and Sunke [52]			√
Sagan and Sobotka [53]				√
Hendriks and Te Dorsthorst [63]				√
Gálvez-Martos, Styles, Schoenberger and Zeschmar-Lahl [54]				√
Kartam, Al-Mutairi, Al-Ghusain and Al-Humoud [55]				√

Table 2

Conventional connections for material composition.

Connection			Reference
Type	Position	Method
Physical fixing	OSB sheathing—timber frame	nails	[75]
	steel module—module	bolts	[76]
	prefabricated concrete beam-column	steel plates, top angles, steel threaded rods	[77]
	prefabricated concrete wall- wall	steel U-shaped channels, bolts, nuts	[78]
	timber roof—wall	toe-nailed	[79]
	timber beam—column	mortise-tenon	[80]
Chemical fixing	brick—stone(within masonry wall)	high-grade mortar	[81]
	steel beam—column	welding	[76]
	steel module—module	welding	[76]
	prefabricated concrete beam—column	steel connector (consists of bolting steel tubes, steel plates), casting	[82]
	concrete plate—beam	steel rivets, casting	[83]
	timber roof—wall	high-strength fibre reinforced polymers with epoxy resin	[84]

Table 3

Tools and methods for material decomposition. (Reproduced with permission from Table 7.1 in ref. [89], which was published by Woodhead Publishing Limited, 2013).

	Tool/Method	Function/Detail
Tools for soft stripping	wrench	loosening bolts, separating metal elements
	bow saw	precise cutting of small wooden elements
	pickaxe	breaking stone or brick sections
	pliers	cutting cables, dismantling metallic and plastic elements
	de-nailing gun	firing small ram to punch nails out
	hydraulic scissors	precise dividing of metal, plastic, wood, brick and concrete
Methods for hard stripping	thermal processes	materials are fused and separating structures
	abrasive processes	the constructed elements are cut into smaller pieces with abrasive saws
	chemical processes	using of highly expansive chemicals, causing fragments of constructed elements
	electrical processes	discharging through elements, resulting in breakage
	explosive	the collapse of a structure with explosives at a critical point of the structure

Table 4

Wall details and treatments.

	Wall	Element	Connection	Separation	Material	Designation
A [105]	[Image omitted. Please see PDF.]	6 mm timber board9 mm OSB20 mm XPS9 mm OSB	epoxy resin	hard stripping	mixed waste(timber, OSB, XPS, epoxy resin)	energy recovery
B [106]	[Image omitted. Please see PDF.]	Φ170 mm timber columns50 × 150 mm timber beams230 × 110 × 50 mm bricks	beams and columns are joined by mortise-tenon; mortar as an adhesive between bricks	soft stripping	timber	reuse
				hard stripping	mixed waste(bricks, mortar)	recycling
C [107]	[Image omitted. Please see PDF.]	50 mm concrete50 mm steel and polystyrene board50 mm concrete	casting	hard stripping	mixed waste(concrete, polystyrene)	disposal
					separate steel	recycling
D [108]	[Image omitted. Please see PDF.]	50 mm concrete board12 mm CSB89 mm steel frame12 mm CSB0.5 mm steel covering50 mm concrete board	screws	soft stripping	concrete	reuse
					steel	reuse
					CSB	unknow
E [109]	[Image omitted. Please see PDF.]	10 mm sand and lime coating24 mm hemp concrete and timber frame20 mm sand and lime coating	concrete is sprayed onto the timber frame	hard stripping	mixed waste(timber, concrete, sand, lime)	disposal

Table 5

Designations of different materials.

Material	Connection	Separation	Designation	Wall
Timber	physical fixing	soft stripping	Reuse	B
	chemical fixing	hard stripping	Energy recovery	A
			Disposal	E
Concrete	physical fixing	soft stripping	Reuse	D
	chemical fixing	hard stripping	Disposal	C/E
Steel	physical fixing	soft stripping	Reuse	D
	chemical fixing	hard stripping	Recycling	C

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